A problem has been presented with unauthorized telephone calls being made at home or work, particularly because of the high costs associated with the charges for normal long distance telephone calls and with the toll charges for "1-900" telephone calls.
Many attempts have been made to solve this problem. Specifically, there are many mechanical attachments available which position a lock in the fingerholes of a dialer on a rotary telephone, place a cover over a telephone keypad, or disconnect the keypad/dialer altogether. Additionally, U.S. Pat. No. 4,092,500 discloses a mechanical attachment that may be positioned on a rotary dialer and on a touchtone keypad. These attachments mechanically inhibit a telephone call from being made by simply preventing a user from actually pressing a touchtone button or by preventing the rotary dialer from turning properly.
Mechanical devices for preventing telephone calls are not preferred because of their appearance and difficulty of disabling. Particularly, the mechanical devices are not feasible for touchtone telephones since an owner will not want to place a mechanical attachment on the keypad, whether it be covering the whole keypad or just some of the buttons. An owner will not also want to disconnect his keypad altogether since it is difficult to turn the keypad "off" and "on" while still preventing a potential maker of unauthorized calls from doing the same.
Other attempts have been made at preventing telephone calls by electrical devices. For instance, U.S. Pat. No. 3,681,536 discloses a device comprising a series of latches which is operable in response to the first digit dialed. If the first digit dialed is an unauthorized digit, a disabling circuit shorts the telephone lines together so that any further digits dialed from the telephone are prevented from being transmitted to the telephone system. Unfortunately, this device can only block calls beginning with the predetermined unauthorized digit. Additionally, a telephone can have more than one long distance telephone company providing service to it. Therefore, this device will not block all long distance telephone calls because the first digit for long distance service now varies from "0" to "9" with the advent of these large numbers of long distance telephone companies.
U.S. Pat. Nos. 4,000,380 and 4,012,602 teach another type of electrical circuit which prevents an outgoing telephone call. This circuit only works on touchtone telephones by counting the number of dialed digits and, if over a predetermined number, the circuit prevents further touchtone signals from being sent to the telephone system. The disadvantage of this arrangement is that it does not block all telephone calls, but only those over a certain number of digits dialed length (e.g. eight or more for a typical long distance telephone call).
U.S. Pat. No. 4,297,534 discloses an electrical apparatus which disables an outgoing telephone call by filtering the signals on the telephone line. In particular, a passive notch filter, tuned to a specific touchtone frequency, attenuates signals at and around this frequency so that the touchtone signal is difficult to detect. Unfortunately, all the dialed digits cannot be filtered unless a series of passive notch filters is provided, each tuned to one of the various touchtone frequencies. Such a series of notch filters greatly increases the size and cost of the device. Moreover, the notch filter or filters, being passive devices, will attenuate all signals at or near its tuned frequency, whether it be a dialed digit, voice, or data signal. Thus, this device degrades voice and data signals. This nonselectiveness in application is highly undesireable.
German Patent No. DE 3,619,165 A1 discloses a further attempt to inhibit an outgoing telephone call on pulse telephones by an R-C filter circuit. This circuit induces a time delay in the rise of voltage on the telephone line when pulse dialing begins. This delay is sufficiently long to cause this signal to be shorted across the telephone line by a transistor. Such a device will not operate properly with a touchtone telephone.